As filed with the Securities and Exchange Commission on October 11, 2012

Securities Act File No. 333-56203

Investment Company Act File No. 811-08797

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

x

UNDER

THE SECURITIES ACT OF 1933

Pre-Effective Amendment No.

¨

Post-Effective Amendment No. 28

x

and/or

REGISTRATION STATEMENT

UNDER

THE INVESTMENT COMPANY ACT OF 1940

x

Amendment No. 72

x

(Check appropriate box or boxes)

BLACKROCK SMALL CAP GROWTH FUND II

OF BLACKROCK SERIES, INC.

(Exact Name of Registrant as Specified in
Charter)

100 Bellevue Parkway

Wilmington, Delaware 19809

(Address of Principal Executive Office)

Registrants Telephone Number, including Area Code (800) 441-7762

John M. Perlowski

BlackRock Small Cap Growth Fund II

of BlackRock Series, Inc.

55 East 52nd Street

New York, New York 10055

United States of America

(Name and Address of Agent for Service)

Copies to:

John A. MacKinnon, Esq.

Benjamin Archibald, Esq.

Sidley Austin LLP

BlackRock Advisors, LLC

787 Seventh Avenue

55 East 52nd Street

New York, New York 10019-6018

New York, New York 10055

It is proposed that this filing will become effective (check appropriate box)

x

Immediately upon filing pursuant to paragraph (b)

¨

On (date) pursuant to paragraph (b)

¨

60 days after filing pursuant to paragraph (a)(1)

¨

On (date) pursuant to paragraph (a)(1)

¨

75 days after filing pursuant to paragraph (a)(2)

¨

On (date) pursuant to paragraph (a)(2) of Rule 485.

If appropriate, check the following box:

¨

This post-effective amendment designates a new effective date for a previously filed post-effective amendment.

Title of Securities Being Registered: Shares of beneficial interest, par value $.0001 per share.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all of the requirements for effectiveness of this Post-Effective
Amendment to the Registration Statement pursuant to

Rule 485(b) under the Securities Act and has duly caused this Registration Statement to be
signed on its behalf by the undersigned, duly authorized, in the City of New York and the State of New York, on October 11, 2012.

BLACKROCK SMALL CAP
GROWTH FUND II

OF BLACKROCK
SERIES, INC.

By

/s/ JOHN M.
PERLOWSKI

John M. Perlowski

President and Chief Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to its
Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.

Signature

Title

Date

/s/ JOHN
M. PERLOWSKI

John M. Perlowski

President and Chief Executive Officer

(Principal Executive Officer)

October 11, 2012

/s/ NEAL J.
ANDREWS

Neal J. Andrews

Chief Financial Officer (Principal Financial and Accounting Officer)

October 11, 2012

DAVID O.
BEIM*

David O. Beim

Director

RONALD W.
FORBES*

Ronald W. Forbes

Director

DR. MATINA S.
HORNER*

Dr. Matina S. Horner

Director

RODNEY D.
JOHNSON*

Rodney D. Johnson

Director

HERBERT I.
LONDON*

Herbert I. London

Director

IAN A.
MACKINNON*

Ian A. MacKinnon

Director

CYNTHIA A.
MONTGOMERY*

Cynthia A. Montgomery

Director

JOSEPH P.
PLATT*

Joseph P. Platt

Director

ROBERT C. ROBB,
JR.*

Robert C. Robb, Jr.

Director

TOBY
ROSENBLATT*

Toby Rosenblatt

Director

KENNETH L.
URISH*

Kenneth L. Urish

Director

2

FREDERICK W.
WINTER*

Frederick W. Winter

Director

Paul L. Audet*

Paul L. Audet

Director

HENRY
GABBAY*

Henry Gabbay

Director

*By:

/s/ BENJAMIN
ARCHIBALD

October 11, 2012

Benjamin Archibald (Attorney-in-Fact)

3

SIGNATURES

BlackRock Master LLC has duly caused this Registration Statement of BlackRock Small Cap Growth Fund II of BlackRock Series, Inc. to be signed on its behalf by the undersigned, thereunto duly authorized,
in the City of New York and the State of New York, on October 11, 2012.

BLACKROCK MASTER SMALL
CAP GROWTH

PORTFOLIOOF
BLACKROCK MASTER LLC

By

/s/ JOHN M.
PERLOWSKI

John M. Perlowski

President and Chief Executive Officer

Pursuant to the requirements of the Securities Act of 1933, this Registration Statement has been
signed below by the following persons in the capacities and on the dates indicated.